1. Field of the Invention
This invention relates to multi-stage fibre amplifiers which are used to amplify optical signals by means of laser action which occurs in the core of the fibre.
2. Related Art
It has long been recognised that fibre amplifiers are an attractive alternative for amplifying optical signals. For example fibre amplifiers have been proposed for use as booster amplifiers to increase the output power from a telecommunications optical transmitter or at a telecommunications repeater to amplify optical signals which have been attenuated by transmission through a first telecommunications link in order that said signal has sufficient power for onward transmission through a second telecommunications link. A fibre amplifier comprises a lasing species, e.g. a rare earth such as praseodymium (Pr), neodymium (Nd) or erbium (Er), which is located in the core of a fibre waveguide. The fibre is connected, usually by means of a wavelength division multiplexer (WDM), to a pump which is adapted to provide pump radiation which interacts with the lasing species to produce the inversion which is needed to support lasing activity. In some embodiments the WDM is connected at the input end of the fibre whereas other embodiments connect the WDM at the output end; i.e. both configurations are known.
The gain available from a single stage amplifier, i.e. as described above, is limited. This limitation is due to a variety of technical features and two important contributors will be identified. In the first place, the power available from a single pump is limited and this limits the lasing activity so that only limited gain can be achieved. In addition, it is difficult to stimulate a sufficient quantity of the lasing species if all the pump radiation is derived from a single source. It has been recognised, at least theoretically, that these limitations could be overcome by the use of a plurality of pumps each of which stimulates a different region of the lasing species. This configuration can be regarded as a plurality of fibre amplifier stages connected in series whereby the output of one amplifier stage forms the input to the next amplifier stage. It will be appreciated that a multi-stage amplifier makes the limitations mentioned above less restrictive on the composite structure. Because there is a plurality of pumps the total power is not limited by the performance of any one of the pumps. Furthermore each pump is only required to stimulate a portion of the lasing species so the problems of distributions are made less onerous.
Many embodiments employ only two stages of amplification and it has been found that the two stages often provide an adequate gain. A simple version of a two stage fibre amplifier takes the form of a single region of suitably doped ("active") fibre connected at both ends by WDM to two different pumps. In other words, the amplifier is pumped from both ends. The two pumps of such amplifiers are also referred to as a co-pump and a counter-pump.
Unfortunately, such co- and counter-pumped multistage amplifiers give rise to difficulties which arise from the interaction between signals in the various stages. It is an object of this invention to enhance the performance of multi-stage amplifiers employing co-pumping and counter-pumping together.